In a typical virgin pulp papermaking process, the pulp, after pulping, is subjected to a series of cleaning operations to remove debris and larger particulate material. The pulp is then subjected to a washing process to remove the dissolved solids generated by the chemicals used in the pulping process. In the past, virgin pulp has been washed using either a drum washing process or a belt washing process. In a drum washing process, a drum having perforated outer sections is partially submerged in the pulp and a vacuum drawn on the inside of the drum, causing a relatively thick layer of pulp to adhere to the outer surface of the rotating drum. As the drum rotates, the mat or layer of pulp is showered with water and the vacuum on the inside of the drum draws the water through the mat. In a typical washing operation, three to five stages of drum washing are utilized.
In a belt washing process, a mat of virgin pulp is fed onto an endless porous belt or fabric and wash water is sprayed onto the mat. The fabric is dewatered by vacuum boxes located beneath the fabric. In a typical operation, five stages of washing may be employed in an attempt to remove the dissolved solvents from the virgin pulp. After washing, the virgin pulp may still contain a significant quantity of dissolved solids, and because of the nature of washing utilizing very thick pulp mats, dirt and fiber debris are also present in the conventionally washed virgin pulp.
Recycled pulp, composed of used corrugated containers, waste paper, white office waste and the like, cannot successfully be washed in the same manner as virgin pulp, because of the presence of inorganic fillers, such as clay and calcium carbonate, in the recycled pulp.
In the conventional drum and belt washing processes, as described above, the pulp mats are relatively thick. Because of the thickness of the mat, it is difficult to remove small particles of the fillers, since the pulp mat acts like a filter and entraps the filler particles.
Consequently, pulp washing of recycled fiber has been done with a side hill screen or unit known as a "Variosplit". With the use of the side hill screen, the pulp is diluted with a tremendous quantity of water and is flowed over the screen. While a side hill screen will act to remove dissolved solids as well as fillers, unfortunately a substantial quantity of small fibers are carried through in the liquid stream. To remove the inorganic filler and small fibers from the liquid stream, a large flotation clarifier is normally utilized.
In the clarifier, air and a coagulative polymer are added to the liquid, resulting in a sludge that floats to the top of the vessel and typically contains from about 60% to 80% of potentially usable fibers, with the remainder being short fibers and fillers. This sludge is further dewatered and traditionally has been landfilled.
Additionally, since only one or two stages of this type of washing are typically employed a large quantity of wash water is required in order to effect a significant reduction in dissolved solids.
Thus, the conventional process for washing recycled pulp not only utilize a tremendous quantity of water, but result in a substantial loss of good fiber material. Further, washing of recycled pulp has been very inefficient in removing dissolved solids, since significant quantities of wash water are required.